Options for treatment of poor-prognosis or relapsed acute myeloid leukemia (AML) remain limited. Gemtuzumab ozogamicin (Mylotarg, Wyeth-Ayerst, Philadelphia, PA) is an immunoconjugate composed of recombinant humanized murine anti-CD33 antibody linked to calicheamicin, a potent cytotoxic agent. Phase II trials have shown the efficacy of gemtuzumab ozogamicin in the treatment of relapsed AML. Trials exploring this agent in other CD33+ hematologic malignancies and in combination with other agents for AML are ongoing. Gemtuzumab ozogamicin is associated with acceptable toxicity as a single agent. However, the incidence of veno-occlusive disease of the liver remains a concern when this agent is used in combination with chemotherapy or in the context of hematopoietic stem cell transplant.